Prince of Wales
(The). This title arose thus: When Edward I. subdued Wales, he promised the Welsh, if they would lay down their arms, that he would give them a native prince. His queen having given birth to a son in Wales, the new-born child was entitled Edward, Prince of Wales; and ever since then the eldest son of the British sovereign has retained the title.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894